Brendan Chen
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74 lines
2.4 KiB
Python
74 lines
2.4 KiB
Python
# Make a calculator using Reverse Polish Notation. Use recursion.
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# The Reverse Polish syntax completely removes the need for parentheses.
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#
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# Syntax examples:
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# "3 + 2" -> ["3", "2", "+"]
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# "3 * 4 + 2" -> ["3", "4", "*", "2", "+"]
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# "3 * (4 + 2)" -> ["3", "4", "2", "+", "*"]
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#
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# Example for how to evaluate ["3", "4", "2", "+", "*"]:
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# Start with "4 2 +" -> "6"
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# Then "3 6 *" -> "18"
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# Start with the "innermost" expression and work backwards (this is the recursion part)
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#
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# For help, email me@bchen.dev
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# Sample solution below
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# Facts:
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# The last element of the list must always be an operator; otherwise there is only one element
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def rpn(expression: list[str]) -> int:
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operators = "+-*/"
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def eval(expression: list[str]) -> tuple[int, int]:
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"""
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Evaluate RPN and return an index value pointing to the first token in the expression.
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"""
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for i in range(len(expression) - 1, -1, -1):
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token = expression[i]
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if token in operators:
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right, right_token_index = eval(expression[:i])
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# for left, we need the index of the token that comes after right
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left, left_token_index = eval(expression[:right_token_index])
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if token == "+":
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return left + right, left_token_index
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elif token == "-":
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return left - right, left_token_index
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elif token == "*":
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return left * right, left_token_index
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else:
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return int(left / right), left_token_index
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else:
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# evaluate as a literal
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literal = int(token)
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return literal, i
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return (0, 0)
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return eval(expression)[0]
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# The following test cases should pass (please copy this into your code):
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tests = [
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(["3", "2", "+"], 5),
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(["3", "4", "*"], 12),
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(["3", "4", "2", "+", "*"], 18),
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([], 0),
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(["50", "2", "/"], 25),
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(["5", "2", "/"], 2), # for division, cast to int using int(num)
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(["5", "4", "3", "-", "*"], 5),
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(["5", "4", "6", "2", "10", "+", "*", "-", "-"], 73),
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(["3", "2", "+", "5", "*"], 25)
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]
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for case in tests:
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result = rpn(case[0])
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expected = case[1]
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if result == expected:
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print(f"PASS: expected {expected}, got {result} for {case[0]}")
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else:
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print(f"FAIL: expected {expected}, got {result} for {case[0]}")
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